1. Field of Invention
The present invention relates to an integrated circuit and a method of fabricating the same; more particularly, the present invention relates to a high voltage device that has a pad oxide with a uniform thickness and a method of fabricating the same.
2. Description of Related Art
Metal oxide semiconductor transistor device is the most fundamental unit in a semiconductor device. A typical metal oxide semiconductor transistor includes a gate, a gate oxide layer and source/drain regions. The gate is configured on a substrate, using a gate oxide layer for insulating from the substrate. The source/drain regions are doped regions having the same types of dopants and are configured in the substrate beside two sides of the gate.
A typical method in forming the gate oxide layer is via performing a thermal oxidation process, in which the substrate surface is exposed to oxygen ambient and a silicon oxide layer is formed thereon. However, the oxidation rate at the substrate surface varies. Hence, the thickness of the resulting silicon oxide layer becomes non-uniform. More particularly, the thermal oxidation rate at the substrate surface of the center area of the active region is higher, while the thermal oxidation rate at the substrate surface of the border between the active region and the isolation structure, especially at the corner of the isolation structure, is lower due to their physical structures. Hence, the resulting oxide layer is thicker at the center of the active region and is thinner at the periphery of the active region. For a high voltage device, for example, for a high voltage device of a fully depeleted metal oxide semiconductor transistor (FDMOS), the target thickness of the gate oxide layer is large. Therefore, differentiation in the thickness between the center and the periphery of the active region becomes even more significant. During the period when high voltage is being applied to the gate, a thin gate oxide layer easily induces a breakdown, especially a time-dependent dielectric breakdown (TDDB). Ultimately, the reliability of the device is seriously affected.